React Native Error Invariant Violation Module AppRegistry is not a registered callable module Occured

BUNDLE  ./index.js 

 ERROR  ReferenceError: Can't find variable: flex
 ERROR  Invariant Violation: Module AppRegistry is not a registered callable module (calling runApplication). A frequent cause of the error is that the application entry file path is incorrect.
      This can also happen when the JS bundle is corrupt or there is an early initialization error when loading React Native.
 ERROR  Invariant Violation: Module AppRegistry is not a registered callable module (calling runApplication). A frequent cause of the error is that the application entry file path is incorrect.
      This can also happen when the JS bundle is corrupt or there is an early initialization error when loading React Native.

And My Index.js

/**
 * @format
 */

import {AppRegistry} from 'react-native';
import App from './App';
import {name as appName} from './app.json';

AppRegistry.registerComponent(appName, () => App);

I did 'kill -9 nodePID' and npm start --reset-cashe and re-run emulator. But my ios-emulator shows white screen. PLEASE HELP ME~~ Thank you.